1. Field of the Invention
The present invention relates to a photomask used in a lithographic process, a method for producing the same, and an apparatus for producing the same.
2. Description of the Background Art
Hitherto, a photomask wherein a shading pattern (semitransparent film) is formed on a surface of a transparent substrate has been used in the lithographic step for semiconductor devices. In the shading film pattern of this photomask, a clear defect such as a pattern chipping or a pinhole may be generated.
In this case, a focused ion beam (FIB) is radiated onto the clear defect in the atmosphere of organic gas, thereby correcting the clear defect. According to this correction, a carbon film whose transmissivity is about 0% to the exposure wavelength used when the photomask is used in a lithographic step is deposited on the clear defect portion. By the carbon film, the clear defect portion is corrected.
For example, in the correction (repaired) of a clear defect in a half tone (abbreviated to “HT” hereinafter) mask for devices which is used at an exposure wavelength of 248 nm and has a design size of 150 nm, an FIB radiation apparatus wherein the accelerating voltage thereof is 20 KeV is used to form a carbon film having such a thickness that gives a transmissivity of about 1% or less to light having a wavelength of 248 nm. However, the correction working accuracy, that is, the location accuracy of the carbon film to be formed in the clear defect portion is, at best, within the range of about ±50 nm from a target location.
In an HT mask for devices which has a design size of 130 nm, the correction working accuracy of a clear defect portion needs to be within the range of about ±30 nm from a target location in order to suppress the dimensional fluctuation of a pattern on a wafer into a tolerable range. Therefore, sufficient working accuracy cannot be obtained according to the above-mentioned FIB radiation apparatus. As a result, about the location, size and shape of a pattern wherein the correction portion of a clear defect is transferred onto a wafer, a dimensional error of pattern on a wafer is generated to such a degree that a bad effect on device performance is produced. For example, in the case of a wire pattern, the wire may break.
Thus, a new apparatus for correcting a clear defect has been developed instead of the above-mentioned FIB radiation apparatus. In this correcting apparatus, the accelerating voltage thereof is made high up to 30 KeV so that the diameter of the beam becomes smaller. As a result, the working accuracy for correcting a clear defect is improved. Accordingly, about the location, size and shape of a pattern wherein the correction portion of a clear defect is transferred onto a wafer, a dimensional error of the pattern on a wafer is not generated to such a degree that a bad effect on device performance is produced.
In an HT mask, the phase effect, which is the effect of reversing the phase of light transmitted through its HT film and the phase of light transmitted through the quartz transmission portion of its transparent substrate by 180°, is used to make the optical intensity of the contour of the pattern edge portion of the HT film the conspicuous. In other words, interference between light transmitted through the HT film and the light transmitted through the quartz transmission portion of the transparent substrate is used to improve pattern resolution between the pattern of the HT film and the pattern of the quartz transmission portion of the transparent substrate.
For this reason, in the case that a carbon film having a transmissivity of about 0%, about which the phase effect is not utilized as in the above-mentioned defect correcting method, is used to correct a clear defect, the pattern resolution as can obtained when the HT film is used cannot be obtained.
After transferring the pattern on the photomask onto a wafer, a dimensional error (transfer error) of a pattern on a wafer is generated about location, size and shape. In a dimensional error, the correction potion of a clear defect on the photomask is contained. On the basis of the dimensional error of a pattern on a wafer, a working accuracy tolerance margin, which is a margin which is tolerable for the location, size and shape of the correction portion of the pattern on the photomask, is decided. In the above-mentioned correcting apparatus, a shading film is used in the correction portion. Therefore, the working accuracy tolerance margin is smaller than in the case that an HT film is used in the correction portion.
Furthermore, patterns of semiconductor devices have been made minuter and wavelengths to which wafers are exposed have been made shorter; therefore, a dimensional fluctuation tolerance margin, which corresponds to working accuracy of the location, size and shape of a shading pattern itself on a photomask, has been made small. With this, the above-mentioned working accuracy tolerance margin of a correction portion is also made small. As a result, the working accuracy of correction portions in the above-mentioned defect correcting apparatus cannot satisfy the working accuracy (which is) required (necessary) to correct a defect on HT masks.
As described above, according to the conventional method of correcting a clear defect in an HT mask (i.e., a clear defect on a substrate of an HT mask), the clear defect is corrected by the deposition of a carbon film about which the phase effect cannot be used for any defect correction portion, the carbon film having a transmissivity of about 0% to an exposure wavelength. Therefore, the working accuracy tolerance margin of the correction portion becomes small which causes an error in the size and shape of a device pattern transferred on a wafer (the error resulting from the location accuracy, the edge shape and the skirt-trailing of the carbon film formed on the photomask). The error negatively impacts the performance of the semiconductor device.